254 THE ELECTRIC TELEGRAPH. 



passing between a and b will encounter only one unit of 

 resistance. In the same way, if the point g y midway between 

 a and b, be put in short circuit with either a or b, the current 

 will meet on its way between a and b with only half the total 

 resistance of the wire. In the same way the intermediate 



Fig. 129. 



points, g and h, being connected by a shunt, the resistance 

 between the ends will be equal to the sum of that between 

 a and g and that between h and b. The resistance of the 

 shunt in each case being infinitely small, its resistance does 

 not appear in the result. 



By this means a considerably greater length of wire can 

 be made use of, and the body of the wire be protected by a 

 case or otherwise, the points c, d, g, h, &c., only being neces- 

 sarily at the command of the operator. 



A highly useful arrangement, for this purpose, is shown 

 in Fig. 130, where the various points in question are con- 



Fig. 130, 



nected to a series of brass terminals, e, /, g, &c., so placed 

 that, by inserting a metallic wedge or plug between any two 

 of them, the length of wire contained between them is short- 

 circuited. If a plug be thus inserted in the hole between e 

 and /, for instance, the resistance d will disappear the 

 current passing through the plug, the resistance of which is 

 infinitely small. 



Between the two first terminals, a and e, a length of wire, 

 whose resistance is equal to one unit upon the reel b, is 

 inserted; between e and /upon the reel^, twice that length ; 

 the same between / and g, and so on, in the following 

 order : 



